I think that this problem has something to do with excerpts. If I fill the excerpts of all the posts, feed validator says the feed is correct. But if any of the posts lacks its excerpt, Feed Validator doesn't validate. To check this assumption, I've made another test with two posts with excerpt and the first one ("Hola, mundo"), without it. In this case, Feed Validator says:

It's obvious that something is broken in the feed. I'm not programmer at all, but I think that this problem has something to do with Sharedaddy, because it occurs with other themes and other blogs, in different circumstances.